Although methane is abundant, its relative inertness has limited its utility in conversion processes for producing higher-value hydrocarbon. For example, oxidative coupling methods generally involve highly exothermic and potentially hazardous methane combustion reactions, which frequently require expensive oxygen generation facilities, and produce large quantities of environmentally sensitive carbon oxides. In order to overcome some of these difficulties, there has been considerable effort directed toward methane conversion via catalytic oxidative coupling reactions.
One process for producing ethylene from methane by catalytic oxidative coupling is disclosed in Synthesis of Ethylene via Oxidative Coupling of Methane, G. E. Keller and M. H. Bhasin, Journal of Catalysis 73, 9-19 (1982). Although an appreciable selectivity to ethylene was observed (to a maximum of about 50%), conversion was relatively low. In order to overcome the methane-ethylene separation difficulties resulting from the low methane conversion, technology has been developed for quenching the reaction product downstream of the oxidative coupling reactor, and then separating ethylene from the unreacted methane.
One process, disclosed in Enhanced C2 Yields from Methane Oxidative Coupling by Means of a Separative Chemical Reactor, A. E. Tonkovich, R. W. Carr, R. Aris, Science 262, 221-223, 1993, includes a simulated countercurrent moving-bed chromatographic reactor, and achieves 65% methane conversion and 80% selectivity to C2 hydrocarbons. The reactor is configured in four sections, with each section comprising (i) a catalytic reactor containing Sm2O3 catalyst and (ii) an adsorbent column located downstream of the catalytic reactor. Methane and oxygen react via catalytic oxidative coupling in the reactor at a temperature in the range of about 900° K to 1100° K, and then ethylene is separated from unreacted methane in the sorption column. In order to maintain sufficient selectivity for ethylene sorption, the reactor's product is quenched to a temperature of 373° K in the sorption column. In another process, disclosed in Methane to Ethylene with 85 Percent Yield in a Gas Recycle Electrocatalytic Reactor-Separator, Y. Jiang, I. V. Yentekakis, C. G. Vayenas, Science 264, 1563-1566, 1994, gas recycle is utilized to further increase methane conversion, but an even lower quench temperature (30° C.) is used during ethylene sorption.
Catalytic processes have also been proposed for co-converting methane and one or more co-reactants to higher hydrocarbon, such as aromatics. For example, U.S. Pat. No. 5,936,135 discloses reacting methane at a temperature in the range of 300° C. to 600° C. with (i) a C2-10 olefin and/or (ii) a C2-10 paraffin in the presence of a bifunctional pentasil zeolite catalyst, having strong dehydrogenation and acid sites, to produce aromatics. The preferred mole ratio of olefin and/or higher paraffin to methane and/or ethane in the feed ranges from about 0.2 to about 2.0.
Other processes utilize organic oxygenate as a co-reactant for the non-oxidative methane conversion to produce higher hydrocarbon, including aromatics. For example, U.S. Pat. No. 7,022,888 discloses a process for the non-oxidative conversion of methane simultaneously with the conversion of an organic oxygenate, represented by a general formula: CnH2n+1OCmH2m+1, wherein C, H and O are carbon, hydrogen and oxygen, respectively; n is an integer having a value between 1 and 4; and m is an integer having a value between zero and 4. The methane and oxygenate are converted to C2+ hydrocarbon, particularly to gasoline range C6-C10 hydrocarbon and hydrogen, using a bifunctional pentasil zeolite catalyst, having strong acid and dehydrogenation functions, at a temperature below 700° C.
However, since the co-reactants employed in the processes of the referenced patents are themselves valuable commodities, there is interest in developing alternative routes for the conversion of alkane, e.g., C4− alkane, such as methane, into C5+ hydrocarbon such as aromatics. Processes which would allow more methane to be incorporated into an aromatic product are particularly desired.
Improvements in distribution of oxygen for carrying out the conversion of alkanes such as methane into C5+ hydrocarbons are also desired. Additionally, reduction in undesired combustion reactions competing with the alkane conversion is desired.